People have known about PTSD since the Napoleonic wars.
Back then it was called nostalgia and was seen as a serious problem.
Later in WWI it was called shell shock and was believed to be partially caused by artillery shells exploding close to you. But because both sides were in desperate need of more man power it was not seen as a reason to back out of the war, and the people who tired to leave because of PTSD were often shot for being cowards.

So true. This is why Vietnam has such a high amount of soldiers with mental issues. Being shunned for what your country sent you to do makes you question why you bothered in the first place to fight in their name.